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Antenna Issues and Technologies for Enhancing System Capacity 179

services and equipment. The 3G services were designed to offer broad-
band cellular access at the targeted speeds of 2 Mbps indoors. The IMT-
2000 operates in the 2-GHz band. NTT DoCoMo, which took over the
mobile communication business from NTT in 1992, started the first
commercial Wideband Code-Division Multiple-Access (W-CDMA) 3G
network in the world in 2001, which made it possible for users to employ
one mobile terminal using a worldwide standard system. The 3G system
is much more powerful in providing multimedia, personalized, and glo-
balization services. The 3G system operates in the 800-MHz, 1.7-GHz,
and 2-GHz bands for increasing system capacity.
Currently, the research and development of mobile communica-
tions in Japan are focused on advancing Long Term Evolution (LTE,
Evolved UTRA, or UTRAN in the 3G band) and the fourth-generation
5,6
(4G) system. The LTE, which is also called Super-3G, allows the 3G
system to work with the 4G system, which is called the IMT-Advanced
system in Japan. In May 2003, NTT DoCoMo conducted a field trial of a
4G mobile communication system in Yokosuka, Japan, with a prelimi-
nary/temporary license.

5.1.2 Wireless Access System

5.1.2.1 Concept The main traffic carried by mobile communication
systems until the 2G system was developed was voice communications.
However, the 3G system requires high-speed transmission service
because this system has changed to data communications. To realize
the high data-rate transmission in 3G networks, a variety of wireless
access systems have been investigated. Figure 5.2 shows the concepts
of 2G and 3G frequency reuse systems in Japan. In this concept, each
base transceiver station (BTS) has been designed to cover a single cell.
Figure 5.2a shows an example of a PDC system, specifically, the 2G
digital cellular telephone communication technology used in Japan. In
the PDC system, one frequency channel is assigned to the individual
target cell, which is different from those used in the surrounding cells,
for avoiding co-channel interference.

F6 F7 F5 F1 F2 F1 F1 F1 F1 F1
F2 F4 F3 F6 F7 F1 F1 F1 F1 F1
F5 F1 F2 F4 F3 F1 F1 F1 F1 F1
F3 F6 F7 F5 F1 F1 F1 F1 F1 F1

(a) PDC repetitive frequency (b) IMT-2000 repetitive frequency
Figure 5.2 Wireless access system